Common medium multichannel exchange system



E. l. GREEN Bec. 22, 1936.

COMMON MEDIUM MULTICHANNEL EXCHANGE SYSTEM B'Shee'ts-Sheet 1 Filed May 2e, 1934 Alll, Aw mm Comman/ .Medium www/sisef wwwp From - Selecting Circuit Z'f Greef/b INVENTOR ATTORNEY e .nm .0/2 z Ff 1 mf ,u ol/.0 tc C #omc w En d @F 0c C Y AQU W0 l d 6 e L Ma E Dec. 22, 1936. E, GREEN 2,064,904

COMMON MEDIUM MULTICHANNEL EXCHANGE' SYSTEM ATTORNEY Dec. 22, 1936.

E. 1. vGREEN COMMON MEDIUM MULTICHANNEL EXCHANGE' SYSTEM Filed May 26, -3.934.

8 Sheets-Sheet 3 NSE@ INVENTOR Z- Green BY ATTORNEY E. l. GREEN Dec. 22, 1936.

COMMON MEDIUM MULTICHANNEL EXCHANGE SYSTEM vFiled May 26, 1934 8 Sheets-Sheet 4 y Dec. 22, 1936. E, l, GREEN A V 2,064,904

COMMON MEDIUM MULTICHANNEL EXCHANGE SYSTEM Filed Hay 26, 41934 8 Sheets-.Sheet 5 rp INVENTOR El 67@ el@ @ffpc ATTORNEY E. l. GREEN COMMON MEDIUM MULTICHANNEL EXCHANGE SYSTEM Filed May 26. 1934 8 Sheets-Sheet 6 mm aw ,Nt mm n m @h m, ma ,obb 00 EIT ofRZ?.

INVENTOR El Green BY fwy ATTORNF-v Dec. 22, 1936. E GREEN 2,064,904

- coMMoN MEDIUM MULTICHANMEL EXCHMMGE SYSTEM Filed May' 26, 1934 s'sheets-sheet 7 25 ther Selecting Cat5.

Ufffce 7@ 0l' er' Channels).

INVENTOR F 77. l I leen/ WL Al IORNFY Dec. 22, 1936. E, l. GREEN A 2,064,904

COMMON MEDIUM MULTIQHANNEL EXCHANGE SYSTEM Filed May 2e, 19:54 a sheets-sheet s F a' ment Y INVENTOR Glce'el/b ATToRNEY Patented Dec. 22, 1936 l'rar ori-ICE coMMoN MEDIUM MULTICHANNEL EXCHANGE sYs'rEM Application May 2s, '1934, serial No. 727,802

23 Claims.

'I'his invention has for its object the provision of an exchange system in which communication between subscribers is carried out over channels of different frequency derived from a common 5 guiding medium. More particularly, the invention is designed to utilize as a substitute for the -many subscriber pairs which are employed in the ordinary ltelephone exchange system the many channels which are obtainable in the high-frequency art as it is now unfolding and to provide `means whereby these channels may be selected by subscribers at will and employed for interconnection.

In the telephone exchange systems which have heretofore been provided it has been the practice to group subscribers in central office areas. Each subscriber is connected to his own central oilice by means of an individual wire circuit or by a party line arranged for non-simultaneous use by ;0 a few subscribers. When a subscriber desires to call another subscriber he utilizes his own line to the central office, and upon passing the desired number either to an operator or to an automatic switching mechanism is connected to another line ,5 which extends either. directly or via another office or oillces to the called party. For a system of this kind many thousands of subscriber circuits are necessary in each central ofice area.

These are commonly provided in the form of a 0 network of multiple pair cables extending over the area.

In accordance with the present invention it is proposed to substitute for such individual wire circuits extending to each subscriber a transmission medium which is common to all subscribers and which will accommodate a range of frequencies suiiiciently wide for a large number of telephone channels. Interconnection .between subscribers is then carried out over these highfrequency channels.

v The essence of the invention resides in making available to a large number of subscribers a transmission medium capable of accommodating a wide' range Iof frequencies so that the subscribers mayy 0 vention may be obtained by ut izing either radior transmissionin space or'high-frequency transmission over a conducting or guiding medium.

"dium for transmitting the wide band of frequencies contemplated by the invention, the in vention is designed to employ whatever part of the frequency range may be available. Heretofore, however. the radio frequency spectrum has scarcely afforded suicient frequency space to yield the channel capacity required for a local tele- 5 phone exchange system. With the opening up of the field of ultra-short waves this limitation as to channel capacity is removed. Thus, for example, between one and two meters there is a total frequency range of 150 megacycles while 10 the range between and 20 centimeters is 1500 megacycles. The frequency ranges obtainable through the use of still shorter waves are much greater.

With these ultra-high frequencies, however, 16 nature imposes another limitation, i. e., the tendency of-the waves to take on quasi-optical characteristics, so that they project only in straight lines and do not bent around corners to any -considerable extent. Thus, it becomes neces- 20 sary that subscribers utilizing such waves have a fairly clear line of sight between their antennas. Some defraction around obstacles may. of course, be obtained, but, in general, reasonably clear lineof-sight projection without intervening obstacles appears to be called for. This, of course, has the advantage of making it possible-to utilize the same ultra-high frequency in different areas.

The line-of-sight characteristics of the ultrashort waves must be recognized in the design of a .30 telephone exchange system utilizing this technique. In one form of the invention it is proposed to provide in the center of a community a central station with antennas mounted sufficiently high above the surrounding buildings to make possible the necessary directness of transmission between these antennas and practically any point in the territory to be served. One subscrlbers station is then placed in connection with another by transmitting first to the central station and 40 through the central station to the desired other subscriber located in.another part of the community. I'he central station besides serving as av switching point acts as a means for catching the waves from the subscribers at a relativelyfhigh point, thus keeping the transmission path above any obstacles lying between the subscribers antennas. f

In order that, such a central station may be economical, it is proposed to makeit capable o f handling simultaneously a wide band of frequen- Y, cies such ,as vwould accommodate hundreds .o r l. Consideringyrst, the use of radio as the m'eperhaps thousands of channels. The particu- 'lar arrangement of the central station will be hereinafter described.

As an alternative to the use of a. radio transmission medium, the invention contemplates the employment of a guiding or conducting transmission path suitable for the required band of frequencies. Such a transmission path would be provided in the form of a network, extending to all the subscribers within a given area.. Conceivably, such a network might be composed of ordinary wire circuits branched and interconnected so as to make the common circuit available to each subscriber. Thus, the wire network might be somewhat similar to the metwork emplayed for distributing light and power currents, except that a simple two-wire network without multi-phase connections, voltage transformation arrangements, etc., would suillce. It would, however. be possible to carry out the invention by employing the light and power network itself and this is contemplated within the scope of the invention.

The preferred form of guiding transmission medium, however, is one which is designed for the transmission of a wide band of frequencies with comparatively low attenuation and preferably also with shielding to minimize the effect of external disturbances. Such a transmission medium may be found in a circuit consisting of two conductors disposed coaxially with respect to one another. In a properly designed coaxial circuit, as will be hereinafter explained. low attenuation at high frequencies may be attained by the employment of conductors of suitably low high-frequency resistance and the use of a substantially gaseous dielectric between the conductors. With this type of circuit the outer conductor serves also as a shield whose protective eect becomes more nearly perfect as the frequency is increased, so that at high frequencies practically complete immunity from external disturbances may be obtained.

The invention contemplates also as an alternative to the coaxial circuit a high-frequency transmission medium comprising a circuit conslsting of two conductors surrounded byv a shield.

by a conductor.

Here, again, the high-frequency attenuation may be minimized by proper design with substantially gaseous insulation and the thickness of shield may be determined so as to minimize interference from external disturbances.

Yet another form of transmission medium suitable for carrying out the invention is a dielectric wave guide comprising a cylindriform dielectric material which may or may An advantageous form of such a. transmission path isa hollow cylindrical conductor containing air or some other gas as the dielectric. This type of circuit is particularly adapted to transmit waves of very high frequencies, for example, of the order of a few centimeters or less in wave length.

'Using any of the above types of transmission media. the interconnection of subscribers at will from the available channels involves a number of new and difficult problems which are solved by' the various features of the invention as hereinafter disclosed.

One of the big functions involved in local exchange telephone systems is that of switching so that each subscriber may be connected with any other subscriber. This operation is now carried out at the central oilices by the switching of mechanical contacts either as a manual operation or by automatic equipment under remote control of the subscriber.- Actually. a great many of these contacts are involved and there is a considerable ot be surrounded p hazard in poor contacts giving rise to microphonic noises in subscribers' lines. Also, the making and breaking of contacts gives rise to surges of currents in these lines due to the presence of dii-eciJ current with consequent noises and clicks.

In accordance with certain embodiments of the invention, it is proposed to carry out this function of interconnecting subscribers without the switching of mechanical contacts. This is accomplished by having the subscribers permanently connected to the common medium and completing the electrical path either directly by thc subscriber at his own premises or at a central omce by frequency selection. Thus, an important object of the invention is the provision of means whereby mechanical switching may be replaced by "electrical switching.

In general, the invention contemplates the assignment to each subscriber of a particular frequency or frequencies which might preferably be -i related to his telephone number. The assignment of frequencies should be such as to facilitate the interconnection of subscribers and various schemes of frequency allocation are provided in the invention for this purpose.

The foregoing outline having indicated some of the principles of the invention as well as a few of its major objects, the complete invention with its various details, features and purposes will lnow be understood from the following description when read in connection with the accompanying drawings, in which Figure l shows a schematic representation of the principal idea of the invention wherein a large number of stations are connected to a common transmission medium;

Fig. 2 shows in schematic form a system for connecting subscribers through a central oice using space radio transmission; Fig. 3 shows one possible allocation of frequencies for a multi-channel exchange system; Fig. i shows a subscriber station arrangement which may beI used in the systemof Fig. 2; Fig. 5 shows a modulating device which may be used in various applications of the invention; Fig. 6 and Fig. 7 in combination show a central oiice arrangement which may be used in association with the subscriber station arrangement of Fig. 4; Fig. 3 shows a cord circuit arrangement alternative to that of Fig. 7; Fig. 9 shows another subscriber station arrangement for the system of Fig. 2; Fig. l0 shows a method of channel selection alternative to that employed in Fig. 9; Fig. ll shows a central oillce arrangement which may be used in association with the subscriber station arrangement of Fig. 9; Fig. l2 shows a frequency adjusting mechanism which may be used in the arrangement of Fig. l1; Fig. 13 shows a method of channel selection which may be used in place of that of Fig. 12; Fig. 14 shows yet another channel selecting arrangement which may be used in connection with Fig. ll;

4 Fig. l5 shows another central ofiice arrangement subscribers in different areas through central oi-l iices over trunk circuits; Fig. 18 is a schematic diagram of a guiding or conducting network which is available in common to a central station and a number of subscribers; Figs. 19. 20 and 2l show different forms of transmission paths which may be used to form a transmission medium common i sary selecting circuits.

omce and the called subscriber.

to a number of subscribers; Fig. 22 shows how the subscriber station arrangement of Fig. 4 might be adapted to a coaxial 1circuit network; Fig. 23 shows an arrangement in which a number of subscriber stations may communicate with one another through a central station over a common transmission network; Fig. 24 shows one scheme for making a vcommon transmission network available to a number of subscribers; and Figs. 25, 26 and 2'7 show network arrangements alternative to that of Fig. 24.

Referring to Fig. l, there is illustrated one of the principal aspects of the invention. In this figure is shown a common medium CM, to which are connected a plurality of stations S1, S2. S3, etc., each of which includes transmitting, receiving and associated apparatus. This apparatus, which will be hereinafter described, is arranged to permit intercommunication between different pairs of stations over the common medium CM employing different frequency bands to permit simultaneous communication between different pairs of stations.` Connection between any pair of stations as, for example, S1 and Sa, may take place through the common central station CS.

Fig. 2 shows in schematic form a telephone exchange system in which a number of subscribers may be interconnected through a central ofce, the connection between each subscriber and the central oice being accomplished byradio transmission through space. Each subscriber is provided with transmitting apparatus including an oscillator-modulator and receiving apparatus including a demodulator, together with the neces- At the central ofce a plurality of circuits, such as the one shown, is provided for interconnecting between subscribers. Each connecting circuit includes two separate ,sets of apparatus, each of which comprisesA a radio transmitter and receiver, and a connection, to which the operator has access, between the two sets of apparatus.

Transmission from subscriber A to subscriber B is effected by means of the transmitter ST1, the

L oscillatonmodulator SM1, the tuned circuit or filter SF1, the antenna SA1, the antenna CA1, the tuned circuit or filter CF1, the demodulator CD1, the hybrid coilOHi, the connecting circuit CC (across which is bridged the operators set CS). the hybrid coil CH2, the oscillator-modulator CM2, the tuned circuit or filter GF4, the antenna CA2, the antenna SA2, the tuned circuit or lter SF4, the demodulator SD2 and the receiver SR2. Transmission in the opposite direction is accomplished in an analogous manner, employing the transmitter ST2, the oscillator-modulator SM2. the selecting circuit SFs, the antenna SAz, the antenna CA2, the selecting circuit OFs, the demodulator CD2, the hybrid coil CH2, the connecting circuit CC, the hybrid coil OH1, the oscillator-modulator CM1, the selecting circuit OFz, the antenna CA1, the antenna SA1, the selecting circuit SF2, the demodulator SD1 and the receiver SR1.

It is proposed that different frequencies be em.- ployed for the oppositely directed transmissions in Fig. 2 and that the frequencies employed between the calling subscriber and the central oce be different from those used between the central Transmission might be on either a double or a single'sideband basis with the carrier frequency transmitted.

In order that simultaneous conversations may take place between a number of pairs of subscribers, it is necessary that the frequencies employed in the system of Fig. 2 be properly selected. One way of arranging the frequencies would be to assign to each subscriber a separate pair of frequencies. one for transmitting and one for rey ceiving, and to provide apparatus at the central oice for receiving and transmitting the various required frequencies. Thus, subscriber A might be assigned a transmitting frequency f1 and a receiving frequency f2, and subscriber B transmitting and receiving frequencies .f2 and f4, respectively. -This would mean that in Fig. 2 the modulators SM1 and SM2 would operate with carrier frequencies f1 and f3, respectively, and the modulators CM1 and CM2 with carrier frequencies f2 and f4 respectively.

With such a scheme it might prove desirable, in order to facilitate the allocation of frequencies and the interconnecting of subscribers, to have a constant difference between the transmitting and receiving frequencies of each subscriber or, in

other words, to make f1f2=f3f4=D The frequencies f1. facto., might then be grouped in one part of the frequency spectrum and the ments corresponds to that of the subscriber numbers. This scheme, while not essential to all embodiments of the invention, is a very desirable feature in many.

Detailed arrangements of apparatus which might be used at the subscribers station and at the central oce in a scheme of this sort are illustrated in Figs. 4 and 6. While some of the appa ratus shown in these and succeeding figures is more particularly adapted to radio frequencies lying below the ultra short-wave range, it will be understood that such apparatus is shown merelyv to illustrate the principles of the invention, and is not intended in any way to limit its scope.

The operation of the subscribers apparatus illustrated in Fig. 4 is as follows: When the subscriber desires to make a call, the receiver SR1 is removed from the switchhook SH. Through the contacts of the switchhook this closes a circuit which furnishes power supply to operate the oscillator and modulator OC and MC. The operation of the switchhook also provides D. C. supply for the transmitter ST1, this supply being derived through a rectifier RF which is connected to the A. C. supply. The speech currents produced in the transmitter, after passing through y the transformer TR1, are modulated upon -the assigned frequency, here assumed to be f1. The oscillator and modulator may be separate units of any desired types, or a self-oscillating modulator might be used. The carrier and Isidebands in the modulator output, after passing through the selecting circuit SF1, are radiated by the antenna SA1 to the central cnice. When communication with the central oflice is established in a manner hereinafter to be described. the number of the called subscriber is passed orally to the operator and after the proper arrangements are set up at the central office, communication is established with the called subscriber who is provided with apparatus identical with that of Fig. 4 except for the frequency assignments used.

In the case of an incoming call, the carrier frequency f2 and its associated sidebands are received by the antennaASAi, selected by the selecting circuit SF2 and demodulated by the demodu- 75 lator SD1. This demodulator may be of the vacuum tube type, in which case it should be kept always in operating condition so that a call may 'be received. Preferably, however, the demodu' rectied. current resulting from the incoming carrier frequency and which in turn operates the call bell SB. When the subscriber answers the call, the switchhook SH is operated, which, in addition to completing the contacts previously mentioned, operates the slow-release relay Ria, which breaks the circuit of the bell SB. IThe received voice-frequency currents, after passing through the transformer TR2, are appliedto the receiver SR1 and two-way communication is thus established. If desired, a low-pass filter LP (shown dotted in Fig. 4) may be included in the receiver circuit to suppress unwanted high-frequency components in the dei'nodulator output. It will be understood that a similarlter may be provided in the arrangements hereinafter described. When the subscriber hangs up, the

slow-release feature of RLz prevents his bell from l ringing before the other party also hangs up.

lt will be noted that the lters SF; and SF2 in Fig. t are arranged to select a xed frequency. Consequently with the exception of one switchhook contact there will be no switched contacts in the transmission path of the subscribers apparatus of Fig. a. 'I he switchhook contact may be readily avoided by replacing the connection through the switchhook by a. permanent connection to the rectierRF as shown by the dotted line y-y. in this case the connection at the point .r--r should be broken.

Figs. 6 and 7 illustrate central oice apparatus which may be used in conjunction with the subscribers apparatus shown in Fig. 4. In the arrangement of Fig. 6 a number of selecting circuits OFi, OEB,y etc., corresponding to the transmitted `irequencies of the different subscribers are connected to a common antenna GA3. The total number or such selecting circuits is equal to the total number of subscribers to be served in the area. Similarly, selecting circuits OFz,

ORL etc., which correspond to the receiving frequencies assigned to the various subscribers are connected to the antenna @A4, if desired., the two antennas might be replaced by a single antenna, or the transmitting and receiving apparatus might be grouped in other ways for connection to the antennas.

The operation of the apparatus in Figs. 6 and 7 is as follows: The incoming carrier frequency oi a calling subscriber is received on the antenna GA3, selected by one of the selecting circuits, for example, CF1, and demodulated by the demodulator ODi. Associated with the demcdulator is a relay ORi whose winding is connected to the demodulator output through the contacts of another relay ORz. The rectied current resulting from the incoming carrier frequency operates the line relay OR1, which in turn lights the line lamp OL1, thereby calling the attention of the operator to the incoming call.

Ihe operator is provided with a number of cord circuits of the type shown in Fig. 7, .each

cord 'circuit terminating in plugs, such as CP1 andOPz.' Upon observing the line lamp she inserts the plug OPi associated with one of the cord circuits into the jack-OJ1 of the calling subscriber. Thus the voice-frequency currents delivered aoeaooa circuit serve to interrupt the D. C. path, so that line and supervisory relays and lamps may be similarly operated at the other side of the connection.

To the conjugate terminals of the hybrid coil Ol-Ii there are connectedl the oscillator-modulator CM1 and the selecting circuit OFz so that the operator may answer the'incoming call. The subscriber then passes the number of the called party to the operator. A busy test of the called subscribers line is obtained through the tip of the plug at the other end ofthe' cord circuit and the sleeve of the subscribers jack. In case another cord circuit is already connected to a jack corresponding to the number of the called party, the sleeve of the jack will have battery connected to it and a click will be produced in the operators receiver. a

lf the called subscribers line is not busy, the operator places the plug CP2 in a jack, such as OJz of Fig. 6, of the called party, thereby completing the connection. The operation of the relay ORs connects power supply to the oscillatormodulator OM-z, so that the carrier is transmitted and the bell of the called party rings. lWhen the called party answers, the supervisory lamp OL: is lighted by means of relay OR4 and remains lighted until the called party hangs up. The

conversation then proceeds with the apparatus her set to different cord circuits, and using the key 0K1 she may split the connection and talk to either subscriber without the other.

It will be noted that the cord circuit connection between the two subscribers in Fig. 7 is arranged on a two-wire basis. If desired, this connection might be made instead on a four-wire basis, using, for example, the arrangement shown in Fig. 8. in this case the plugs OPa and OPi each establish four contacts in addition to that of thesleeve, and the jacks OJs and OJ.; are substituted for the jack and hybrid coil combinations of Fig.. 6. The oppositely directed paths ofthe connection are brought through the splitting key 0K1. The operator may talk to either or both subscribers by means` of the key 0K1 and the hybrid coil CH3 which is connected through the key 0K2 to the operators transmitter and receiver OT and OR. The connections of the line and supervisory lamps and relays and other minor details are not shown in Fig. 8, it being understood that their operation will be generally siml ilar to that described in Figs. -6 and 7.

amplification of the signals may be included in the central oiice connection.

The total number of frequencies or` charme 75 required for the scheme just described is evidently equal to twice the number of subscribers. The

scheme is therefore somewhat wasteful of central i oce apparatus inasmuch as it requires a separate modulator, demodulator and selecting circuits for each pair of subscribers frequencies.

I n order to reduce the amount of apparatus required at the central oflice, various alternative methods of frequency allocation might be used in place of the one just described. In accordance with one such method one pair of frequencies would be assigned to each subscriber for incoming calls. In addition, a number of pairs of frequencies equal to the maximum number of conversations that might be desired at any one time would be set aside for use by calling subscribers `in reaching the central oiice.V In placing a call,

the subscriber would select, either manually or automatically, an idle pair of frequencies over which to establish connection to the central office. He would then pass the number of the called party to the operator, who would complete the connection to a jack corresponding to the number of the called party, and this operation would adjust the frequencies of the other half of the cord circuit to those of the called subscriber. Details of apparatus which might be used at the subscribers station and the central oiiice for carrying out this method are shown in Figs. 9, 10. ll, l2, 13 and 14.

The operation of the arrangement shown in Fig. 9 is as follows: When the subscriber desires to make a call, he removes the receiver SR1 from the switchhook SH. The operation of the switchhook `provides D. C. supply for the transmitter ST1 and the power supply for the oscillator and modulator OC and MO in the manner already described. In addition, the operation of the switchhook energizes the slow-release relay RLz whose winding is connected to the D. C. supply through one of its own back contactsl and back contacts of the relays RLa and RL1. (At the time the switchhook is removed the relay RL1 in the demodulator circuit is deenergized, since the demodulator is connected through one of the back contacts of the relay RLS, which is also deenergized, to the fixed selecting circuit FFz which is used for incoming calls.) Relay RLz is ay slowrelease relay which remains pulled up during the entire selecting operation. The condenser RC which is bridged across the winding of RLz contributes toward the slow-release feature.

The operation of the relay RLz energizes the slow-release relay RLa, which is connected through the make contact of RLz, one of its own break contacts and the switchhook. The relay RL: in operating establishes for itself a new path direct through one of its own make contacts and the switchhook, so that this relay remains in operation until the receiver is vreplaced on the hook at the conclusion of the call.

The operation of relay RLa serves to connect the modulator MO and the demodulator SDi through the selective switching apparatus SW to the variable selecting circuits VFr and VF2, which in turn are connected to the antenna SAI.

Before continuing with the description of thearranged in the form of a semi-circle, so that wipers may be operated step-by-step by the stepping magnet RLs to engage the various contacts in succession. A double-ended wiper is provided for each row of contacts so that switching takes place always in a forward direction. The different banks of contacts are all arranged to be operated from the same stepping magnet.

Each of the variable filters VFi and VFz comprises a T network which includes in each series arm a tapped condenser, such as TCi, and a fixed inductance, such as F11. The shunt portion of each lter comprises in parallel a xed condenser, such as FC1, and a tapped inductance, such' as TL. 'Ihe tapped condensers TC1 and TG2 and the tapped inductance TIi are connected, respectively, to the banks of contacts s1, sa and s2 so that when the wipers of these banks are in any given position, the lter is adjusted to select a particular high-frequency channel (passing both the carrier and its sidebands). The proportioning of the tapped condensers and inductance is such that the filter VF; is able to select any one of ten channels assigned for transmitting from the calling subscriber to the central oiiice, while the iilter VF2 is able to select any one of ten channels assigned for transmitting from the central office to the calling subscriber. It will be understood that the showing of ten pairs of channels for establishing connection with the central office is purely arbitrary and that any other desired number might be provided instead, the total number being such as to accommodate the maximum number of conversations expected to take place simultaneously.

While it would be possible for the subscriber to select an idle pair of channels by manual adjustment, it is considered preferable to perform this selection automatically, and the switching apparatus SW, in combination with the variable filters VFi and VFz, accomplishes this result. The method by which this selection is carried out will now be explained. If, when the relay RL: operates, the switching apparatus SW is connected to an idle pair of channels, the wipers of the switching unit remain in position, and the channel is used by the subscriber for obtaining his connection through the central omce in a manner hereinafter to be described. If,` however, at the time the relay RLa is operated, the switches are connectedv to a pair of channels already in use, a carrier frequency is received by the demodulator SD; and the resulting rectified current operates the relay RL1. This completes a circuit through the winding of the relay RL4, which is connected through a make contact of the relay RLi, a make contact of the relay RLz, and a back contact of the stepping magnet RLs. The relay RIA through its make contact completes a circuit which energizes RLS, and the operation of the stepping magnet RLs advances all wipers by one position.

The operation of the stepping magnet RL5 also deenergizes the relay RL.; which is connected through the back contact of RL5. This in turn deenergizes RLS. If at this time the switching unit is connected to an idle pair of channels, no further operation occurs and the subscriber is ready for communication with the central oice.

If, however, the connection has been made to` another busy pair of channels, the relay RL4 will operate as before, and the switching mechanism will be advanced' another step, this operation being repeated until an idle pair of channels is found. The relay RLe, being of the slow-release type, will not release during the channel selecting operation.

The selection of a pair ci channels to the centrai oilce has served in addition to adjust thel frequency of the subscribers oscillator from the frequency normally used for answering incoming calls to the 'transmitting frequency corresponding to the outgoing channel selected. This adjustment is accomplished through the bank of contacts s1 on the switching unit. A The xed condenser FC3, which is connected across the plate winding of the oscillator coil, `produces the frequency for answering incoming calls. The operation of the relay RLa, however, connects the tapped condenser TCs across the plate winding instead, and the adjustment of this condenser is determined so that the frequency corresponds to that of the variable tuned circuits on each switching step.

Upon the completion of the channel selection -Y the slow-release relay RLz releases, and this connects the carrier supply from the oscillator to the modulator through a. back contact of RLz. When the operator at the central oice plugs in to an-l swer the call, a carrier frequency will be transmitted into the demodulator, pulling up the relay RL1. The relay RL4 does not operate now, however, since it is connected through a make contact of the relay Rlla, and no further stepenough for the relay EL3 to,release, since this will break the connection. In case this feature is considered undesirable, the connection of the carrier supply through the switchhook may be re-` placed by a push-button contact with which the subscriber may ash the operator without disturbing the circuit of relay- RLa.

The operationl of the arrangement of Fig. 9 in receiving an incoming call will now be described. When the subscribers receiver is on the switchhook, relay RLa is deenergized and the modulator and demodulator are connected through the back contacts of this relay to selecting circuits FFi and FFz, which correspond to the pair of frequencies permanently' assigned to the subscriber for incoming calls. The oscillator frequency is properly adjusted by the condenser FC3.

The rectified current produced by the incoming carrier. frequency operates the relay RL1, closing the circuitof the bell SB, which is connected through one of the contacts of the switchhook. When the subscriberanswers the call, the bell circuit is broken by the switchhook and the apparatus is in condition for conversation with the calling party. l

It will be understood in connection with Fig. 9 that other switching or adjusting mechanisms night be employed'for selecting an idle channel for use by the callingy subscriber in reaching the central oice.

It will also be understood that other types of variable selecting. circuits and oscillator control -transmitted to this mechanism.

xed condenser FC1 to form a variable iilterl equivalent to the filter VF; in Fig. 9. It will be understood that all of the variable inductances and condensers required might be either mounted on or connected to a common shaft, and'that the banks of switch contacts shown in Fig. 9 would thereby be rendered unnecessary. It will be unde'rstood also that theadjustable or variable inductances and capacitances as described might be employed in other types of selecting circuits.

For the central ofiice apparatus to be used in conjunction with the subscribers apparatus'of Fig. 9, the arrangement of Fig. ll may be employed. Referring to Fig. 11 it will be seen that the apparatus at the left-hand side of the diagram including the cord circuit as far as the operators set and splitting key is the same as that shown in Fig. 6 and Fig. 7. The operation of this part of the central oiice arrangement, which is l-used for the connection with the calling subscriber, will be evident without further explanation.

The right-hand side of Fig. 11 shows in schematic form the arrangement of apparatus for establishing connection with the called subscriber. It will be seen that in this case a modulator and demodulator OMz and OD2 are permanently associated with the cord circuit through the hybrid coil OH2. "I'he oscillator O02 furnishes to the modulator a carrier supply whose frequency is controlled by the selecting mechanism SM. The modulator output is connected to' a -variable channel lter OF5 Whose frequency of response may be adjusted by the selecting mechanism SM. Similarly the demodulator input is derived from the variable channel filter OFs whose frequency is subject' to adjustment by the selecting mechanism SM. The variable channel filters are connected respectively to the antennas OA.; and OAS.

In completing `the connection to the called subscriber'the operator inserts the plug OPz, which is associated with the cord circuit she has used in answering the call, in the jack OJ2 which corresponds to the number of the called party.

'I'his action places the code sending mechanism CS in'connectionwith the selecting mechanism SM and causes a code of impulses corresponding to the number of the called subscriber to be Under the guidance of this code the mechanism SM performs the following simultaneous functions: (1) Adjustment of the carrier frequency delivered to the modulator to that frequency assigned to the called subscriber for the reception of incoming calls; (2) adjustment of the variable channel filter OF5 to pass the sidebands corresponding tothe frequency of thel cord circuit oscillator; (3) adjustment of the frequency of the variable channel filter OFe to the transmitting frequency assigned to the called subscriber'for'use on incoming calls. In addition, if desired, the mechanism SM might be used to adjust the tuning of the antennas as is indicated by the dotted lines extending to OAs and OAs.

Details of one possible arrangement-whereby these various operations may be accomplished are illustrated in Fig. 12. The method of operation of the arrangement shown in this figure is as follows: Before placing the cord circuit plug OPz in the jack OJ2 which corresponds to the number of the called party, the operator makes a busy test, using` the circuit through the sleeve contact of the jack, the upper contact of the plug and thence through her key to the winding of an inductance coil which is` bridged across her receiver. is inserted, and the relay IR1'is thereby energized through the sleeve connection of the jack and plug. The make contact of this relay connects power supply to the motor DM'which drives a wheel TW having a number of small teeth arranged in sets corresponding to the digits in the number of the called party, and having in addition a single large cam. The plug connection has served also to furnish battery to the initial relay FR of the selecting mechanism SM. The rotation 'of the wheel TW interrupts this supply, thereby producing successive sets of break impulses` in accordance with the digits of the subscribers number.

When the wheel has gone through this code, the large cam completes a. make contact which energizes the relay IR2 whose break contact interrupts the power supply to the motor. The relay IR2 locks itself up through the sleeve connection of the jack and plug so that the motor remains stationary while the connection is up. When the operator takes down the connection, the relay IR2 falls back and the mechanism is 1 in readiness for a new connection.

Before tracing the operation of the selecting mechanism SM in response to the code which it receives, it may be pointed out that this mechanism comprises a number of relays associated with two selecting switching units SW1 and SW2 each similar to the one already described in connection with the subscribers apparatus in Fig. 9. In the present case, however, each unit comprises eight .adjacent rows of contacts and eight double-ended wipers all responsive to a single stepping magnet. The stepping magnet MR1 controls the operation of SW1, while MR2 controls the operatic-n of SW2. Each row of contacts in the switching units includes, in addition to ten working or off normal" contacts, a normal contact to which the wiper returns when the operators connection is taken down.

A further feature provided in the switching unit SW1 is a set vof off normal spring contacts ONS of the type familiar in the machine switching art. In the normal position of the switching unit SW1, a lug on the shaft presses down on the off normal springs, closing the contacts on the lower sides. When the unit moves off normal, the pressure on the off normal springs is released and the upper"` contacts are made. v

The circuits controlling the operation of the switching units SW1 and SW2 will now be explained. Upon the establishment of the connectiomthe energizing of the relay FR closes a circuit which pulls-up the,slowrelease relay RR. 'Ihe motor driven wheel now dials the first digit of the subscribers number. example, that this number is 34. Then the circuit of relay FR will first be interrupted three times in fairly rapid succession. 'I'he deenergizing of the relay FR will break the circuit of the relay RR, but since the latter is a slow-release If the line is not busy, the plug Let us suppose, for- This circuit through the relay SR and the stepping magnet MR1 is immediately broken by the reenergizing of the relay FR after the rst break impulse. The stepping magnet MB1 releases, but since SR is a slow-release relay it holds until the next impulse. The second break impulse again deenergizes the relay FR and again closes a circuit through the relay SR and the stepping magnet MR1. This time, however, the circuit is completed over a new path leading from ground over the back contact of the relay FR, the make contact of the relay RR, contacts c and b on the off normal springs (closed when the switch SW1 was advanced one step), the make contact of the relay SR and the stepping magnet MR1. Thus the switch SW1 is advanced another step. The third break impulse acts just as the second one did to advance the switch one more step.

After the third break impulse there will be a time interval before the dialing of the next digit so that the slow-release relay SR will be deenergized. This will break the circuit through this relay so that when the second set of impulses arrives, the stepping magnet MR1 will be out o? the circuit. The rst break impulse of the second series causes the relay FR to fall back and thereby closes a circuit through the break contact of relay FR, the make contact of relay RR, off Anormal contacts c and b, the break contact of relay SR and thence through the winding of the stepping magnet MR2. Thus the wipers of the switching unit SW2 will all be advanced one position. The stepping magnet MR2 will be deenergized at the end of the first break impulse and the following three impulses will act each to advance the switch SW2 by one step just as the rst one. Consequently at the conclusion of the dialing, all the wipers of the switch SW1 will be in position 3 and those of the switch SW2 in position 4. l

The connections of the various rows of contacts s1, s2, sa, etc., of the switching units SW1 and SW2 will now be explained. The two variable b and filters similar to those used in the subscribersstation arrangement of Fig. 9 are employed. Each band filter comprises T network which includes in each series arm a xed inductance and a capacity whose value depends upon the position of the wipers of the switching units SW1 and SW2. 'Ihe shunt part of the T network comprises a xed condenser connected in parallel with an inductance whose value is deter as the case mately one-tenth of those on unit SW1.

inductance V11 and the condenser VC1.

scribers frequencies are -arranged in the same y order as the switch positions (and call numbers),

the values of capacity and inductance associated with the switching units should be arranged in an order of magnitude inverse to the switch position numbers, The steps of inductance and capacity on switching unit SW2 should be approxi- If the capacity and inductance values diier by uniform intervals, the frequencies will evidently be separated by non-uniform intervals.

Certain further features of the arrangement of Fig. 12 will new be described. It will be recalled that the large lug on the toothed wheel TW closes a circuit through the relay IRz and thereby stops the dialing process. This relay lRz also closes a contact which energizes the relay OR. The relay OR, which remains energized as long as the connection is established, through its make contact connects the carrier supply from the oscillator O02 to the modulator OMz.

When the connection is taken down the relay FR is deenergized and this in turn deenergizes the slow-release relay RR. At this time two circuits are closed to ground through the back contacts of relays RR and FR. One of these leads through the wiper of contact row sa of switching unit SW1 to one of the off normal contacts of this row and thence through a back contact of the the wiper of row sa of switching unit SW2, an oli normal contact of this row, a back contact of the stepping magnet MR2 and the winding of this magnet. These circuits cause the stepping magnets MRi and MR2 to buzz around until the normal contact position of cach is reached. Thus the switching units are placed in readiness for the next connection.

If desired, tapped -inductances and condensers similar to those used in Fig. 9 might be employed in place of the individual condenser and inductance units associated with the switching steps in Fig. 12. Another possibility would be to derive the desired values of inductance and capacitance from variablecondensers and inductances which might either be mounted directly on the shafts of the switching units or controlled by them. For this purpose the condensers and inductances to be controlled by the stepping magnet MR1 may be mounted on a common shaft which is provided with a wiper traversing an annular ring. This is shown in the perspective drawing of Fig. 13. The condensers and inductances to be controlled by the stepping magnet MR2 are similarly shown mounted on another shaft. The condensers and inductances controlled by MRi may have values approximately ten times those of the condensers and inductances controlled by MR2. The corresponding units on the two shafts may be suitably connected together and to the proper place inthe tuned circuit.

Referring to Fig. 13, in response to the stepping magnet MR; the shaft is stepped around to a position corresponding to the rst digit of the desired number, thus adjusting the values `of the rst 180 degrees of rotation of the shaft covers the useful range of the circuit elements. Since these elements are continuously variable any acentos The number of teeth on the ratchet Rai should therefore be twice the number of positions employed. For ten digits .the ratchet should have teeth.

Whenl the connection is taken down, a circuit is completed through the stepping magnet and its back contact, the wiper and annular ring, and the back contact of the release relay to ground This completes the revolution of the shaft around i to the starting point where an insulated segment It will be understood, of course, that other.

known types of tuned circuits may be employed in place of the single section band filters shown in Fig. 12.

It will be noted that the arrangement of Fig. 13 eliminates all mechanical contacts in the central omce connection except those ofthe operators plug.

While the arrangement of Fig. 12 is designed to care for a maximum of 99 subscribers, it will be evident that its capacity may be increased by methods familiar to those skilled in the automatic switching art.

An arrangement alternative to that of Fig. l2 and somewhat simpler is shown in Fig. 14. In this case fixed condensers, such as ICi, ICz, etc., and fixed inductances, such as IIi, etc., are associated with the jack OJz, representing the called party. When the operator inserts the cord circuit plug OPz into this jack, the inductances and condensers serve to adjust the oscillator frequency and the frequencies of the transmitting and receiving selecting circuits to the desired values. The same result might also be accomplished by providing the operator with keys correspond ng to the called subscribers, and having her operate a key instead of inserting a plug in a jack.

In comparing the central office arrangement of Fig. 11 with that of Fig. 6,4it will be evident that While there is more apparatus associated with a cord circuit in Fig. l1, the total amount of central oice apparatus is very much less, since the number of cord circuits required will be only a small fraction of the total number of subscribers. On the other hand, the subscriber station arrangement of Fig. 9 is more complicated than that of Fig'. 4, owing to the fact that an idle pair of channels' is selected for the outgoing call.

Another scheme will now be described which may employ a subscriber station arrangement like that of Fig. 4 and a central oilice arrangement in which the apparatus individual to each. subscriber is small. The central oce arrangement for this scheme is illustrated in Fig.i15.v

In this case, a pair of channels` is permanently assigned to each subscriber, one for transmitting and one for receiving. ,The central oflice is arranged so that a call conlng in at any assigned frequency is automatically picked up and brought to the operator, who completes the connection to the called subscriber. Referring to Fig. 15, the antenna OA is de signed to receive incoming carrier frequencies within a range corresponding to all of the subscribers assigned to the central oce or to a group of subscribers. The received frequencies are delivered to the tuned circuit OFo whose frequency of response is being continuously varied by the driving motor DMn. The same motor also rotates the switch SWo which is provided with number of positions may be included in this range. a number of contacts, one for each subscriber 75 whose incoming frequency may be selected by the selecting circuit OFo. The switch is arranged so that the contact corresponding to any given subscriber is made at the same moment that the frequency of that subscriber is picked out by the selecting circuit.

'Ihe carrier frequency of an incoming call is selected momentarily by OFo and is rectied in the rectier ODo. The resultant current operates the relay ORD, which closes a circuit through the winding of the slow-release relay ORi, the back contact of the relay ORz and the switch SWO. The operation of the relay OR1 lights the subscribers line lamp OLi thereby calling the attenticn of the operator to the incoming call. Similarly, as the selection and switching are being varied, theline lamps corresponding to other incoming calls will be lighted.

When the operator observes the lamp OL1 she inserts a plug, for example, OP1,'which is associated with one end of a cord circuit, in the subscribers jack OJ1. This action completes a circuit through the relay OR2, who-se operation deenergizes the relay OR1 and thus extinguishes the lamp OL1. In addition, the insertion of the plug in the jack O31 serves to adjust the response frequency of the variableselecting circuit OFi to the transmitting frequency of the calling subscriber, while at the same time the oscillator O01 and the selecting circuit OFz are brought to the receiving frequency assigned to the calling subscriber. This adjustment of the frequency of OFi, Olli;V and O01, is accomplished by a frequency controlling mechanism FC1 which may be similar to the arrangement illustrated in Fig. 12 or Fig. 14. The incoming carrier frequency of the calling subscriber is rectified in the demodulator OD; and operates the relay ORa,

therebylighting the supervisory lamp CL2 during the time when the calling subscribers receiver is oif the hook.

Having ascertained from the calling subscriber the number which is desired, the operator completes the connection by inserting the other cord circuit plug CP2 in the jack OJz which corresponds to the number of the called party. This action energizes the relay ORB, thereby preventing the line lamp of the called subscriber from lighting when he answers the call. Also, the completion of the connection by means of the frequency adjusting device FCz serves toY bring the frequencies of the oscillator O02 and the selecting circuits OFs and' GF4 to the frequency assignments of the called subscriber. The supervisory lamp OLa is lighted when the called subscriber answers, and remains lighted until he hangs up. 'I he other features of the cord circuit in Fig. 15 being similar to those of previous figureswhich have already been described, no further explanation will be needed.

As noted, the selecting circuit OFo and the switch SWo might serve for all the subscribers assigned to the oice, or these. subscribers might be divided into groups, with one selecting circuit and switch taking care of each group, .and all such circuits and switches driven by a common motor.

It is apparent that the arrangement oi' Fig. 15 permits the employment of a relatively simple arrangement at the subscribers station, inasmuch as the frequencies used by the subscriber are alwaysv the same. The cord circuit arrangement at the central oiiice is somewhat. complicated, but the number of cord circuits required vdevised which employ the principles which have already been disclosed but differ in detail from the arrangements described.

It will be evident that all of the arrangements described'in connection with Figs. 4 to 15 are adaptable to modern manual switchboard practice, including the use of multiple switchboards, and that the lines may be multipled to A operators for answering incoming calls and to B operators for completing' connections to called subscribers. Arrangements for making the busytests required in such multiple operation have been described in connection with certain of the figures and may be similarly obtained in the others.

In the various arrangements shown in Figs. 2 to l5 radio transmission between the subscribi ers and the central oiiice may be accomplishedl directly using any suitable type of antenna. However, in order to permit the use of ultrashort waves,' for which substantially line-ofsight projection maybe desired, it may be necessary to carry the transmission path above any obstacles lying between the antennas of individual subscribers. For this purpose an arrangement of the type shown in Fig. 1 6 might be employed.`

Referring to Fig. 16, it will be seen that in this case a single central oilice antenna 'OA serves for receiving all frequencies from subscribers and transmitting all frequencies to subscribers. 'I'his antenna may be mounted o'n a tall building or tower so that substantially a line-of-sight transmission path is obtained between the central -oiiice and each subscribers antenna within the central oilce area. The subscribers antenna should preferably be designed for radiating a beam toward the central oice antenna and for receiving radiation from that antenna. 'I'hus the subscribers antenna might be a balanced doublet SA as shown in Fig. 16, placed near the focus of a parabolic reflecting surface, PR., directed toward the central oice antenna OA. The central oilice antenna, however, is designedfor radiating to and receiving from all subscribers. For this purpose there might be employed an antenna capable of radiating in all directions and receiving radiations from all directions throughout the range of Afrequencies utilized. Such an antenna, for example, might be a balanced doubletiOAras shown in Fig. 16. Another possibility would be to employ a central ofdce antenna. so designed that itis capable of radiating into and receiving from a conical space which comprehends all subscribers locations. This might be accomplished by adding the reilector RE above the -signed for handling a part ofthe total range of frequencies. It would also be possible to employ two antennas for each subscriber instead of one.

The'invention further contemplates that subscribers stations of the types described above l shall be located in different central office areas and interconnected both with one another and with other types of stations. This may be accomplished by a connection between each subscriber and his central oflice, using a frequency or frequencies assigned` for that' purpose, and completing the connection over suitable interoiiice trunks. The. method is illustrated schematically in Fig. 17. The central ofce apparatus to be used in carrying out this method might be patterned after that shown in previous figures, particularly Fig. 6, 1l or 15. The types of trunks which may be used to interconnect the central oices are well known in the art.

odyne method, with partial selectivity provided' at the channel frequency and the remaining selectivity furnished by a sharp, intermediate frequencyl selecting circuit or filler.

It will be evident to those skilled in the art that arrangements for automatically controlling the volume of the signals delivered by the subscribers equipment may. be incorporated in any of the subscribers stationfarrangements which have been described, so that conversation-between diierent subscribers maytake place over a substantially. constant transmission equivalent.

It will be noted that in all of the subscriber station arrangements described above, the apparatus is designed to prevent a subscriber from listening in on lthe conversations of other subscribers. This, of course, is a very desirable feature for a telephone exchange system.

In all of the arrangements thus far described it has been assumed that free space is employed as the common transmitting medium. There will n'owv be described arrangements in which transmission over a high-frequency conducting or guiding -path is employed instead of radio transmission. Such a path would take the form of a network extending to all subscribers within a given a'rea and to the central omce for that area.

Probably the simplest form of such a network would be one composed .of ordinary two-conductor circuits suitably interconnected and branched so that the common circuit will be available at all desired points. The circuits composing the network might, for example, consist of pairs of open-wire or cable conductors. A network of this kind is shownschematically in Fig. 18,where each line represents a pair of conductors, C

designates the central ofhce and S'a subscribers station.

It is contemplated also in laccordance with the invention that the wire network employed for distributing light and power currents might be used as the common medium from Awhich to derive high-frequency channels for the telephone exchange system. The high-frequency channels would be superposed upon the power network by carrier methods which are well lnown in the art.

Another form of transmission'mediulm, and one which is peculiarly advantageous Vin that it is capable of transmitting a wide band of frequencies with comparatively low attenuation and which may be so shielded as to be practically immune to external disturbances, may be found in a circuit consisting of Atwo conductors disposed coaxially with respect to one another. Such a circuit has been disclosed in the patents to L. Espenschied and H. A. Aiel, No. 1,835,031,

December 8, 1931, H. A. Aicl and E. I. Green,

in the conductors becomes the factor which determines the minimum transmission level. A diagram of one form of coaxial circuit is given in Fig. 19, where i designates an outer cylindrical conductor andl 2 is a conducting wire or tube concentric therewith.' VThe two conductors are maintained in proper spaced relationship by the insulating disks 3. The circuit is shownjconnected to apparatus 5.

As an alternative to the coaxial circuit, the high-frequency transmission medium might comprise a network of circuits, each consisting of two parallel conductors surroundedvby a shield. Cir- -cuits of this type are disclosed in the applications Leibe, Serial No. 674,764, Patent No. 2,034,034 'j all led June 7, 1933 and all patented March 17,

1936. One form of such a shielded pair is illustrated in Fig. 20, where Iv designates a conducting shield and 2 and 4 represent conductors located on opposite sides of and-equidistant from the axis of the shield. These conductors are held in proper spaced relation to one another and the shield by means of insulating disks 3. The conductors 2 and 4 may be connected. to apparatus 5. In a circuit of this type it'is desirable to employ conductors of suitable size, with as far as possible gaseous insulation, in order to minimize the high-frequency resistance and capacitance and thereby reduce the high-frequency'attenuation. The thickness of the shield moreover, should be such as to minimize external disturbances.

n The invention contemplates also utilizing as the high-frequency transmission medium a dielectric wave guide comprising cylindriform dielectric maten'al which mayl or may not be surrounded by a conductor. Such dielectric guides are disclosed in the patent applications of G. C. Southworth, Serial No. 661,154, filed March 16, 1933, and Serial No. 701,711, led December 9, 1933. An advantageous form of such a wave guid@ is a hollow cylindrical conductor containing air or some other gas as the'dielectric. This typ of circuit is particularly adapted to Atransmit waves of very high frequencies, for example, of the order of a few centimeters or less in wavelength. Fig. 21 shows one form of such a wave guide. Apparatus 5 produces high-frequency signaling currents which are propagated along-the coaxial circuit comprising the tube I and the conductor 2, separated from one another by insulating disks 3, as shown in the cut-away portion of the tube. If the frequency is sufliciently high, the conductor 2 need continue only a short distance inside the tube I, and the electromagg5 netic waves from the apparatus travel along conductors l and 2 and will continue along the tube l where the inner conductor has been removed, transmission taking place on the inner surface of the tube and the e'nclosed dielectric. A similar device may. be used at the other end of the dielectric wave guide to receive the signals.

it is contemplated in'accordance with the in l vention that any of the subscriber set and central ofdice arrangements which have' been described may be employed with any of the above types of transmission paths. essary merely to substitute the transmission network for the radio path, replacing the connection of the apparatus to the radio antenna by a connection suitable to the type of medium employed. Thus, for example, Fig. 22 illustrates how the arrangement of Fig. 4 might be adapted to use a coaxial circuit network as the transmission medium. As shown, the inner conductor 2 of the coaxial circuit is connected to the selecting circuits SF1 and SF2 while the outer conductor i may be expanded to form a shield around part or all of the apparatus.

When using a network such as that of Fig. 18 for a system in which connections are established vthrough the central oiiice, there might be some advantage in so allocating the frequencies that the outermost subscribers utilize the lower frequencies for which the attenuations are smaller, while the close-in subscribers utilize higher frequencies which undergo higher attenuations.

Fig. 23 shows an arrangement wherein coaxial lines extend in several directions from the central point. Each line is terminated in a resistance RT which approximates its characteristic impedance. At the central point each of the inner conductors of the various branches is brought through a resistance JR to a common junction point. The function of the resistance JR is to avoid high-frequency reections which otherwise would be produced at the junction. The hybrid coil HB is shown connected to the common junction through a resistance JR, though it would be possible to omit this resistance if a suitable value were assigned to the others. The opposite side of the hybrid is terminated in a resistance BR which is equal to JR plus the parallel impedance of all' the branches. The various transmissions arriving at the central point may be amplified by the amplifier AM before interconnections.

The arrangement of Fig. 23 may be employed with the subscriber station arrangements hereinbefore described in which different carrier frequencies are employed for opposite directions of transmission. The central office apparatus for communication with subscribers is shown vat CA in Fig. 23. In case amplication at the central station is not required, the apparatus CA may be connected direct to the line junction.

There will now be considered' some of the schemes which may be followed in disposing the network which forms the common transmission medium so that it will be available to the subscribers in a given area as well as to the central is to arrange the network in the form of Va grid as illustrated schematically in Fig. 24. In'thi's gure each line represents a conducting circuit or For this purpose, it is nec-Y guide (for example, a coaxial circuit), the various circuits being connected together at points indicated by the dots. The equipment of each subscriber may be connected to the nearest point of the grid through a high impedance, as shown, for example, in Fig. 23. The central station apparatus CS is connected to the grid somewhere near its center.

One' possible difficulty in the arrangement of' Fig. 24 may arise from the large number of points of junction or branching which it contains. Each of these points represents a source of transmission loss due to the division of energy between the branches, and also, unless the junction points are especially treated, a source of high-frequency reflections which may be produced by the improper termination which the junction may oiier to each circuit entering it. A further diiiiculty may arise due to the fact that between any two points of the grid there are v several paths over which transmission may take place, so that the relative phases-of the signals arriving over the different paths may result in very inelhcient transmission for particular frequencies between particular points.

Another type of network arrangement is shown in Fig. 25. Here a single transmission circuit is provided which extends outwardly from the central station in what may be termed a square spiral, the circuit being terminated at its outer end in a resistance RT which is substantially equal to its high-frequency characteristic impedance. Again, each subscriber may be connected to the nearest point of the network, the length of such connection, however; being on the average somewhat greater than for Fig. 24. It will be evident that the scheme of Fig. 25 eliminates the points of branching which appeared in Fig. 24 and that the totalnetwork mileage has been reduced.

Yet another possible arrangement is shown in Fig. 26, this arrangement being similar tothe grid of Fig. 24 but having fewer points of branch` ing. The ends vof the various circuits may be terminated in resistance RT as shown in Fig. 24. It would evidently be possible by bringing the various paths back to the central point to obtain a scheme having only a single junction point rather than the multiple junctions as shown.

Another type of network arrangement is shown in Fig. 27, the circuits being' arranged in the form of main feeders extending outwardly from the central station and branch feeders extending at various points from the main feeders.

While the entire preceding discussion has been' those illustrated without departing from thespirit of the invention as defined in the following claims.

What is claimed is: l

1. In a system whereby a plurality of communi.- cations of intelligence may be simultaneously effected over' a common transmission medium and through a central oce between any one andany other of a plurality of subscribers stations in pairs, the method which consists in transmitting certain frequencies between the subscribers sta-Y tion and the central oiice for establishing connections between calling subscribers stations and the central oilice, and'transmitting certain other frequencies between the subscribers station and the central oce for completing connections from the central oice to called subscribers stations, each of said first-mentioned frequencies being available to all of thc stations.

2. In a system whereby a plurality of telephone communications may be simultaneously eected over a common transmission medium and through a central ofr'ce between any one and any other of a plurality of stations taken in pairs, the method which consists in employing certain frequencies for establishing -connections between calling sta-v tions and the central office, and employing certain other frequencies for completing connections from the central office to called stations, said first-mentioned frequencies being available to all stations, and each of said second-mentioned lfrequencies being available to only one station.

3. In a system whereby a plurality of telephonecommunications may be simultaneously effected over a common transmission medium and through a central office between any one and any other of a plurality of subscribers stations in pairs, the method Iwhich consists in transmitting certain pairs of frequencies between the subscribers station and the central office for establishing connections between calling subscribers stations and the central oiice, and transmitting certain other frequencies between the subscribers station and `the central omce for completing connections ,A from the central office to called subscribers stations, each of said ymst-mentioned frequencies being available to all of the stations.

4. In a system whereby a plurality ofvmore than two telephone' communications may be `simultaneouslg effected over a common transmission medium and through a central office between any one and any other of a plurality of stations taken in pairs, the method which consists in transmitting between a calling station and the central office over any one of a plurality of pairs of frequencies reserved for that purpose, and in transmitting between the central ofce and a called station over a xed pair of frequencies assigned exclusively to that station.

5. In a. system whereby a plurality of telephone communications may be simultaneously effected over a common transmission medium and through a central office between any one and any other of a plurality of stationstaken in pairs,

the method which consists in selecting at a calling station from a. plurality of ainilable pairs of carrier frequencies an idle one of said pairs, utilizing said selected pair of frequencies, one for transmission in either direction, in establishing a connection between the calling station and the central office, and completing the connection from the central office to acalled station by utilizing a fixed pair of carrier frequencies, one

such station, one for transmission and one for reception, for establishing a telephone connection between any station and the central oice, means rendering each station capable of selecting a pair of frequencies which is not in use, and two fixed frequency circuits at each station, one for transspectrum.

7. In a system whereby a plurality of telephone communications may be simultaneously effected over a common transmission medium and through a central'oice between any one and any other of a plurality of stations taken in pairs,

two adjustablefrequency circuits at each such station, one for transmission and one for reception, for establishing a telephone connection be; tween any station and the central oiice, means rendering each station capable of selecting a pair of frequencies which is not in use, and two fixed frequency circuits at each station, one for transmission and one for reception, for incoming calls, each station being assigned a telephone number, the transmitting frequencies of the stau tions being located -in one part of the frequency spectrum and arranged in substantially the same order as the telephone numbers, and the receiving frequencies of the stations being located in another part of the frequency spectrum and arranged in substantially the same order as the telephone numbers.

8. Ina system whereby a plurality of telephone communications may be simultaneously effected over a common transmission medium and through a central office between any one and any other of a plurality of subscribers stations in pairs, means for transmitting between calling subscribers stations and the central oiice over certain carrier frequencies transmitted between the subscribers station and the central office, and means for transmitting between the central oice and called subscribers stations over other carrier frequencies transmitted between the subscribers station and the central omce, each of said rstmentioned frequencies being available to all of the stations.

9. In a system whereby a plurality of telephone communications may besimultaneously eected over a common vtransmission medium and through a central olce between any"one and any other of a plurality of stations taken in pairs, means for transmitting between .calling stations and the central oflice over certain carrier frequencies, and means for transmitting between the central oflice and called stations over other carrier frequencies, said first-mentioned fre.- quencies being available to all stations, and each of said second-mentioned frequencies being available to only one station. f/ 10. In a system whereby a plurality of teleI phone communications may be simultaneously eiected over aA common transmission Amedium and through a central ofce'between any one and any other of a.plurallty of stations taken in pairs, means for transmitting between a calling station and the central office over any one of a plurality of pairs of frequencies reserved for that purpose, and means for transmitting between the central office and a called vstation over a xed pair of frequencies assigned exclusively to that station.'

11. In a system whereby a plurality of telephone communications may be simultaneously 

